1. Field of the Invention
The present invention relates to an exposure mask used for the photolithography process as one of the semiconductor device fabrication processes. More particularly, the invention relates to a Levenson-type phase-shifting mask that suppresses effectively the optical proximity effect to thereby improve the resolution, and a method of forming a pattern using the mask.
2. Description of the Related Art
In recent years, high-speed operation and large-scale integration of semiconductor devices have been progressing further. According to this tendency, it has been required to further miniaturize the patterns of layers that form the devices.
More recently, the design rule has been decreased to approximately half of the wavelength of exposure light (i.e., exposure wavelength). Thus, it is extremely difficult to form small patterns with the size of approximately half of the exposure wavelength by using ordinary exposure methods. To cope with this, various types of “super-resolving technique” have been developed and discussed.
One of the known “super-resolving techniques” is the “phase-shifting mask”. This mask is an exposure mask having a patterned phase-shifting layer selectively formed on the transparent parts (e.g., openings) of the transparent substrate. The patterned phase-shifting layer eliminates the effect of diffraction of exposure light passing through adjoining transparent parts, thereby raising the resolution of the mask.
The “Levenson-type” phase-shifting mask provides much enhancement of the resolution, which is disclosed, for example, in the Japanese Examined Patent Publication No. 62-50811 published in 1987. With the “Levenson-type phase-shifting mask”, a patterned phase shifting layer is alternately formed on adjoining transparent parts of the transparent substrate. This is to make the light beams passing through the transparent parts opposite in phase to each other, thereby suppressing the interference between these two beams. In this way, the mask enhances its resolution.
The “Levenson-type” phase-shifting mask is very effective to enhancement of the resolution and the depth of focus for periodically-arranged pattern elements. This mask can resolve extremely miniaturized patterns with the size of approximately half of the exposure wavelength or less. Therefore, it has been thought that this type of mask is most hopeful as the technique that realizes formation of patterns with the size of approximately half of the exposure wavelength or less.
FIG. 3 shows an example of circuit or element patterns (hereinafter, which are referred as circuit/element patterns) to be formed. In FIG. 3, the circuit/element pattern 110 is used to pattern a conductive film formed on a gate dielectric film, thereby forming the gate electrodes of Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) and the wiring lines connected thereto. The pattern 110 is made of any photoresist film.
The circuit/element pattern 110 includes an isolated pattern section 113 with an isolated, L-shaped pattern element 113a and a periodic pattern section 114 with closely-arranged, linear pattern elements 114a. The isolated pattern section 113 includes the L-shaped pattern element 113a only, in which no other pattern elements are located near the element 113a. The periodic pattern section 114 includes the linear pattern elements 114a that are arranged in parallel at equal spaces or intervals, which is termed the Line and Space (L/S) pattern.
Actually, the circuit/element pattern 110 of FIG. 3 includes various types of other pattern elements than the elements 113a and 114a. However, they are omitted in FIG. 3 for the sake of simplification of explanation.
FIG. 1 shows a prior-art Levenson-type phase-shifting mask used to form the circuit/element pattern 110 of FIG. 3. FIG. 2 shows a prior-art ordinary (i.e., non-phase-shifting) mask (not the Levenson-type) used to form the same pattern 110.
The prior-art phase-shifting mask 120 of FIG. 1 is of the positive type. The mask 120 comprises an L-shaped blocking or light-shielding part 122a for forming the pattern element 113a of the pattern section 113 of the pattern 110 in FIG. 3 and six linear blocking parts 122b for forming the pattern elements 114a of the pattern section 114 of the same pattern 110. The mask 120 further comprises a rectangular phase-shifting part 123a formed closely to the blocking part 122a and three strip-shaped phase-shifting parts 123b arranged alternately in the spaces between the blocking parts 122b. The remaining area of the mask 120 is a transparent part 124.
In FIG. 1, a character “0” is attached to the transparent part 124, because no phase shift occurs in the exposure light passing through the part 124. A character “π” is attached to the phase-shifting parts 123a and 123b, because phase shift of “π (180°)” occurs in the exposure light passing through the parts 123a and 123b. 
The blocking part 122a has the same shape as the pattern element 113a of the circuit/element pattern 110. Each of the blocking parts 122b has the same shape as a corresponding one of the pattern elements 114a of the pattern 110.
The prior-art non-phase-shifting mask 130 is of the positive type, like the phase-shifting mask 120. The mask 130 comprises a rectangular blocking part 132 that covers the blocking parts 122a and 122b of the mask 120 and the phase-shifting parts 123a and 123b thereof. The remaining area of the mask 130 is a transparent part 134. The blocking part 132 has a following relationship with the blocking parts 122a and 122b and the phase-shifting parts 123a and 123b of the phase-shifting mask 120.
Specifically, if the non-phase-shifting mask 130 is entirely overlapped with the phase-shifting mask 120, the upper edge 132a of the blocking part 132 of the mask 130 approximately accords with the upper edges 122aa and 122ba of the blocking parts 122a and 122b of the mask 120. In this state, the upper edges 123aa and 123ba of the phase-shifting parts 123a and 123b of the mask 120 are shifted upward from the upper edge 132a of the blocking part 132 in FIGS. 1 and 2, and overlapped with the transparent part 134.
Next, a method of forming the circuit-element pattern 110 of FIG. 3 using the phase-shifting mask 120 and the non-phase-shifting mask 130 with the double exposure method is explained below.
In the first exposure step, a photoresist film (not shown), which has been formed on an object 112 for pattern formation (e.g., a polysilicon film formed on the gate dielectric film), is irradiated by specific exposure light using the phase-shifting mask 120 of FIG. 1. At this time, a latent image having the same shape as the L-shaped blocking part 122a and the linear blocking parts 122b is formed in the photoresist film thus exposed.
In the first exposure step, an undesired latent image is formed in the photoresist film thus exposed, which is due to the “0-π phase edges” formed at the locations corresponding to the edges 123aa and 123ba of the phase-shifting parts 123a and 123b. 
In the second exposure step, to eliminate the “0-π phase edges”, the photoresist film is irradiated with the same exposure light as used in the first exposure step again using the non-phase-shifting mask 130 of FIG. 2.
Thereafter, the photoresist film including the latent image is developed with a known developing solution, thereby removing the unnecessary, irradiated parts of the photoresist film. Thus, the latent image is elicited, in other words, the photoresist film is patterned as desired. As a result, the circuit/element pattern 110 of FIG. 3 is formed on the object 112 for pattern formation.
With the pattern formation method using the above-described phase-shifting mask 120 and the non-phase-shifting mask 130, however, the following problem will occur.
Specifically, the intensity distribution of the exposure light that passes through the area of the mask 120 where the blocking parts 122b and the phase-shifting parts 123b are periodically arranged is very different from the intensity distribution of the exposure light that passes through the area of the mask 120 where the blocking part 122a and the phase-shifting part 123a are formed. It is thought that this is due to the “optical proximity effect”. As a result, there arises a problem that the minimum size of discriminable or formable pattern elements increases.
FIG. 4 shows the intensity change of the exposing light passing through the phase-shifting mask 120 as a function of the position, where the light intensity is shown with relative values. In FIG. 4, each of the periodically-arranged blocking parts 122b has a width of 0.1 μm, each of the periodically-arranged phase-shifting parts 123b has a width of 0.2 μm, the isolated blocking part 122a has a width of 0.1 μm, and the isolated phase-shifting part 122b has a width of 1.6 μm. The lateral axis of FIG. 4 denotes the distance from the center line of the blocking part 122a or 122b in a perpendicular direction thereto. The position “0” is located on the line.
As seen from FIG. 4, both the exposing light passing through the area corresponding to the isolated blocking part 122a and the isolated phase-shifting part 123a and the exposing light passing through the periodically-arranged blocking part 122b and the periodically-arranged phase-shifting part 123b have increasing intensities with the increasing distance from the center line of the part 122a. However, the increasing rate of the light intensity for the isolated parts 122a and 123a is less than that for the periodically-arranged parts 122b and 123b. Therefore, as shown in FIG. 5, if the inter-element distance (i.e., the distance between adjoining pattern elements) is larger than 0.5 μm (i.e., the pattern element approaches its isolation state), the minimum, formable pattern-element size for the photoresist film increases abruptly.